Threlkeld Microgranite

 

Under plane polarised light the clear patches show anhedral quartz grains set in a groundmass of fine grained quartz and feldspar. Most of the feldspar is sericitised, i.e. converted to muscovite mica.

Under cross polarised light, euhedral plagioclase feldspar phenocrysts with varying degrees of alteration are clearly visible, one towards the top left showing a hint of twinning. The microgranite is reputed to contain garnets but none are seen in this section, the dark patches being quartz grains in the extinction position.

 

The following information is taken from
BRAMCRAG QUARRY,
S.C. Loughlin, OS Grid Reference: NY320220

Volume 17: Caledonian Igneous Rocks of Great Britain

Chapter 4: Lake District and northern England

Site: BRAMCRAG QUARRY (GCR ID: 288)
Extracted from the Geological Conservation Review
You can view an introduction to this volume
at http://www.jncc.gov.uk/page-2731
JNCC 19802007

The age of the Threlkeld Microgranite is around 451 Ma. Its relationship with the Ordovician country rock and other Lake District intrusions has been the subject of debate for many years. The relationships exposed at Bramcrag Quarry are crucial in constraining the further interpretation of the isotopic dates. The Bramcrag Quarry site is the only locality that demonstrates that the Threlkeld microgranite intrudes both the Skiddaw Group and the lowest part of the Borrowdale Volcanic Group. The intrusion is considered, on petrographical and geochemical evidence, to be contemporaneous with the thick ignimbrites of the upper part of the Borrowdale Volcanic Group. The available Rb-Sr age of 438 6 Ma post-dates the cessation of volcanism and may represent either the age of emplacement of the Threlkeld microgranite or a resetting event. Accurate U-Pb dates on zircons may offer a resolution to this problem.

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